1. Field of the Invention
The present invention relates to a method of non-invasively measuring a physical quantity such as a component concentration in a living body, such as a blood-sugar level in a human body, in a non-contact state and an apparatus therefor.
2. Description of the Prior Art
In order to measure a physical quantity in a living body, a measuring apparatus must interact with the surface of the measured body in some form. Also, in case of making measurements through the principle of absorption of light, interaction such as the application of light to the living body is unavoidable. In a measuring method carried out by directly bringing a probe with a sensor into contact with the living body, such contact exerts remarkable influence on the measured object, and hence a measurement error is increased unless such influence is suppressed to a minimum. Specifically, when the measured object is a conscious living thing such as a human, he is not only physically but also mentally influenced by the measuring method. Furthermore, the influence varies with the difference between the equilibrium state of the human and the state of the probe. There is the apprehension that a process for adjusting the equilibrium to a new state causes a disturbance on the measurement of the physical quantity to influence the measured value depending on the difference between the states, whether the human himself is conscious or unconscious.
Settlement of measuring conditions such as the position and the orientation for measurements by bringing a probe into contact with a measured object is being studied. However, no such measuring conditions are settled in a non-contact state, and a method in a contact state cannot be applied to measurements in a non-contact state as such. If the measuring conditions are determined while bringing the measured object into contact with a probe or the like, conscious or physical influence resulting from the contact is exerted on the measured value. In order to avoid such influence, a method of determining measuring conditions in a non-contact state is necessary. Specifically, for example, in case of measuring the concentration of a minor component such as hemal glucose concentration, an obtained signal is extremely weak by nature and physical or mental influence disorders the measured value to deteriorate a signal-to-noise ratio and causes difficulty in extraction of the signal if the measurement is made in a contact state. Such influence resulting from the contact is considered as one of the factors hindering improvement in accuracy of non-invasive glucose measurement at present.
The method of minimizing a measurement error resulting from such physical or mental influence is adapted to arrange the measured object in a space not to be in contact with the probe for measurements by irradiating the measured object with light from a measuring apparatus.
It is known that measurement errors result from fluctuation of relative positional relations between the measured object and the measuring apparatus in the case of making measurements in a non-contact state, although physical or mental influence following contact can be eliminated in this case (refer to Japanese Patent Laying-Open Gazette No. 9-49794 (1997)). Therefore, no accurate measurement result can be obtained unless the measured portion (the position irradiated with measuring light), the measured area (the area of the surface of the measured object irradiated with the measuring light, i.e., the distance between the measuring apparatus and the surface of the measured object) and the measuring orientation (the direction of incidence of the measuring light upon the measured surface) are precisely determined.